Holdoff algorithm for no dead time acquisition

ABSTRACT

An improved hold-off algorithm that assures that all data associated with all trigger events in a data signal are displayed uses a designated interval starting with a first trigger event to determine whether any subsequent trigger events occurred within the designated interval. A first display frame is drawn based upon the first trigger event. A next display frame is drawn based either on a next trigger event that occurs outside the designated interval, or based on the last trigger event that occurred within the designated interval. In the latter case the two display frames provide an overlap to assure that no data related to the trigger events is lost on the display.

FIELD OF THE INVENTION

The present invention relates to an acquisition system for a test andmeasurement instrument, and more particularly to an improved hold-offalgorithm.

BACKGROUND OF THE INVENTION

The “dead time” of a measurement instrument, such as an oscilloscope, isa time period during which data acquisition circuitry does not respondto a valid trigger event because the oscilloscope is busy performingother tasks and so is not able to process trigger events that may occur.Consequently, a waveform representing an electrical signal beingmonitored is not displayed for the missed valid trigger event. In ananalog oscilloscope, for example, dead time occurs during the beamretrace time on a cathode ray tube. In a digital oscilloscope, dead timeoften occurs when the instrument is busy reading data from anacquisition memory associated with a previous acquisition, or busydrawing the acquired processed data to produce an image of the waveformfor display.

Circuits under test often operate at rates much faster than a standarddigital oscilloscope can display the corresponding waveforms. In fact,the typical digital oscilloscope “ignores” most trigger events becauseit is busy processing and drawing waveforms relating to data acquired inresponse to a prior trigger event. It is an unfortunate fact that suchelectronic circuits under test occasionally work in an unexpectedmanner. Occurrences of incorrect operation of the circuits under testmay be rare, perhaps occurring once in thousands of correct cycles ofoperation. Thus, the oscilloscope may not acquire data representingwaveforms that exhibit the incorrect operation of the circuit undertest, i.e., an anomaly, because the oscilloscope may be busy at theinstant that the anomaly occurs. An oscilloscope user may have to wait along time in order to view the incorrect operation. Since only a smallfraction of the waveforms are drawn on the oscilloscope display, failureto observe the incorrect operation cannot give the user confidence thatthe circuit under test is operating properly.

The basic digital oscilloscope has an architecture in which data isreceived and stored in an acquisition memory, and then acquisition ishalted by a trigger event after a defined post-trigger interval. Theacquired data then is read from the acquisition memory for processingand waveform drawing on a display before the acquisition system is againenabled to respond to new trigger events.

Co-pending U.S. patent application Ser. No. 11/388,428, filed by StevenSullivan et al on Mar. 24, 2006 entitled “No Dead Time DataAcquisition”, herein incorporated by reference, is one attempt to enablethe acquisition for display of data representing all trigger events. Ameasurement instrument receives a digitized signal representing anelectrical signal being monitored and uses a fast digital triggercircuit to generate a trigger signal, wherein the trigger signalincludes all trigger events within the digitized signal. The digitizedsignal is compressed as desired and delayed by a first-in, first-out(FIFO) buffer for a period of time (pre-trigger delay) to assure apredetermined amount of data prior to a first trigger event in thetrigger signal. The delayed digitized signal from the FIFO is deliveredto a fast rasterizer or drawing engine, upon the occurrence of the firsttrigger event, to generate a waveform image. The waveform image is thenprovided to a display buffer for combination with prior waveform imagesand/or other graphic inputs from other drawing engines. The contents ofthe display buffer are provided on a display screen at a display updaterate to show a composite of all waveform images representing theelectrical signal.

Two or more drawing engines may be used for each input channel of themeasurement instrument to produce two or more waveform images, eachwaveform image having one of the trigger events at a specified triggerposition within a display window. The waveform images are combined toform a composite waveform image containing all the trigger events forcombination with the previous waveform images in the display buffer orwith graphics from other drawing engines. For certain trigger positionswithin the display window, an indicator is provided to show that atrigger event may have been missed. Also, when there are no triggerevents, a graphic of the signal content may still be provided for thedisplay.

“No dead time” was defined as the ability for the user to see 100% ofthe trigger events that occur within an input signal on the display.Referring now to FIG. 1 locations A-H represent trigger events withinthe input signal. In this example, event C is used as a reference fordrawing the data contained in the left frame, being placed at thetrigger point within the graticule. The above-mentioned '428 U.S. patentapplication discusses a hold-off period within which trigger events arerecognized, but not treated specially. These trigger events are shown ona display to an instrument user as part of the left frame, but do notform the basis for beginning new drawing cycles for these triggerevents. Once the no dead time hold-off period is completed, the nexttrigger event is the focus for a new drawing cycle, in this examplebeing event F which is located at the trigger point within the graticulefor the right frame.

There are some problems with this method of defining “no dead time.” Itis possible for the user to miss important and anomalistic informationeven while seeing 100% of the trigger events. As the trigger point ismoved to the right hand side of the graticule, i.e., the pre-triggerregion of the graticule is increased, more and more parallel drawingprocesses are required to keep up with the incoming trigger events thatoccur just past the right edge of the screen—practically there arelimits to how far the trigger point may be moved toward the right edgeof the graticule. Also drawing the same information more than once onscreen may result in user confusion, i.e., the data around events D andE are drawn on both the left and right frames in the present examplewhich are superimposed when drawn on a display screen, as shown in FIG.1A.

As an example of missing important information, refer to FIG. 2 wherethe user is using edge triggering to look at individual pulses that forma longer pulse train. Reasonably, the user expects the instrumentfeaturing no dead time acquisition to show any anomalies associated witheach of the pulses. However this is not necessarily the case. Each ofthe trigger events, A-D, is shown in either the left or right frame.However, a glitch that is clearly visible in the pulse between triggerevents B and C is not shown in either frame, i.e., it is not shownwithin the graticule on screen, as shown in FIG. 2A, and so is missed bythe user. This condition occurs when the time between a first (A) and anNth trigger event (C) is greater than the sum of the pre- andpost-trigger time intervals that define the graticule; and the N−1trigger event (B) is displayed when drawing the first trigger event. Inthis event a data “dead zone”, as opposed to a trigger event dead zone,may open up between the two processed frames into which informationfalls that is not drawn on the screen, i.e., in this example data withinthe post trigger time interval for the B trigger event but outside thepre-trigger interval for the C trigger event.

What is desired is a no dead time acquisition system that includes alltrigger events and all data within the pre- and post-trigger timeintervals for each of the trigger events.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides an improved hold-offalgorithm for processing data to assure that all data associated withall trigger events within the data are displayed. When a first triggerevent occurs, a first display frame is drawn and a designated intervalis started. If a subsequent trigger event occurs prior to the expirationof the designated interval, then at the expiration of the designatedinterval the last trigger event to occur is used for drawing a nextdisplay frame which provides for overlap between the two display framesto assure that no data is missed related to the detected trigger events.If there are no trigger events during the designated interval, then thenext display frame is drawn when the next trigger event occurs.

The objects, advantages and other novel features of the presentinvention are apparent from the following detailed description when readin conjunction with the appended claims.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a plan view of a signal waveform displayed within twooverlapping display frames according to a no dead time processingalgorithm, and FIG. 1A is the corresponding display screen.

FIG. 2 is a plan view of a signal waveform displayed within twonon-overlapping display frames according to the no dead time processingalgorithm, and FIG. 2A is the corresponding display screen.

FIG. 3 is a graphic view illustrating the improved no dead time hold-offalgorithm according to the present invention.

FIG. 4 is a plan view of the signal waveform of FIG. 2 displayed withintwo display frames according to the improved no dead time hold-offalgorithm of the present invention, and FIG. 4A is the correspondingdisplay screen.

FIG. 5 is a plan view of the signal waveform represented in FIG. 1displayed within two display frames according to the improved no deadtime hold-off algorithm of the present invention, and FIG. 5A is thecorresponding display screen.

FIG. 6 is a block diagram view of circuitry for implementing theimproved no dead time hold-off algorithm according to the presentinvention.

DETAILED DESCRIPTION OF THE DRAWING

Referring now to FIG. 3, a time line is shown about a trigger point thatincludes a pre-trigger region and a post-trigger region, correspondingto the pre- and post-trigger time intervals that define a graticule in adisplay frame. The improved hold-off algorithm, as described herein,starts a time interval from the trigger point that is equivalent to thesum of the pre- and post-trigger time intervals or data window interval.During the data window interval, all triggers that occur after theinitial trigger event, A, represented by the trigger point within thedisplay frame, i.e., trigger events B-F, are recorded in a buffer. Oncethe data window interval expires, a controller determines if there havebeen any other trigger events that occurred during the data windowinterval. If there have been no trigger events after the initial triggerevent, A, within the data window interval, then the next display frameplaces the next trigger event at the trigger point of the next displayframe. Otherwise the next display frame places the last prior triggerevent, F, within the data window interval for the first display frame atthe trigger point of the next display frame. FIG. 4 shows the newhold-off algorithm as applied to the waveform of FIG. 2 where triggerevent B occurs before the completion of the data window interval andprior to trigger event C that occurs after the data window interval. Inthis way the glitch that went undetected under the old hold-offalgorithm is now presented to the user on the display, as shown in FIG.4A.

FIG. 5 is a comparable view to that of FIG. 1, however presenting thetrigger events so the screen display minimizes overlap, as shown in FIG.5A. As is apparent all the trigger events together with theircorresponding pre- and post-trigger regions are presented on thedisplay.

FIG. 6 represents one embodiment of circuitry that enables the use ofthe hold-off algorithm described above. The input signal, whether arealtime signal or a previously acquired signal (in which case the A/Dconverter is bypassed), is digitized by an A/D converter 12 and storedin a data memory 14. The data memory 14 has a capacity that encompassesat least two display frames of data. The digitized data from the datamemory 14 are input to fast rasterizers 16, only two of which are nownecessary to encompass all display trigger positions on the display, fordrawing of the waveform images for the display. A trigger detectioncircuit 18 generates a trigger signal from the digitized data thatencompasses all trigger events within the input signal. Each triggerevent within the trigger signal is time-stamped (20) and the triggertimes are input to a trigger store 22, each subsequent trigger timeoverwriting the previous trigger time.

The first trigger event in the trigger signal, which is positioned atthe designated trigger point within the first display frame, starts aninterval timer 24. The interval timer 24 counts the interval equal tothe designated sum of the pre- and post-trigger delays, i.e., the datawindow interval. The first trigger event in the trigger signal alsoenables a set-reset flip-flop 26, which is reset by the interval timer24 after the data window interval times out. If a subsequent triggerevent occurs in the trigger signal prior to the conclusion of the datawindow interval, the set-reset flip-flop 26 is set. When the data windowinterval times out, a D-type flip-flop 28 samples the output of theset-reset flip-flop 26 to ascertain whether any additional triggerevents after the first trigger event occurred during the data windowinterval. If any additional trigger events occurred, then a flag is setand sent to a controller 30. The controller 30, in response to the flagsignal, takes the trigger event time from the trigger store 22 thatcorresponds to the last trigger event that occurred prior to theexpiration of the data window interval, and uses the extracted triggerevent time for drawing the next display frame. The controller 30 usesthe extracted trigger event time to generate an address for the datamemory 14 from which the data is forwarded to the fast rasterizers 16for drawing the next display frame. In the absence of the flag signalthe controller 30 uses the next trigger event that occurs after theexpiration of the data window interval to draw the next display frame.

A specific embodiment is described for the purpose of illustration only,and the above-described hold-off algorithm may be executed completely bysoftware or by other hardware configurations that produce the sameresult.

Thus, the present invention provides an improved hold-off algorithm forprocessing an input signal to assure that all data within the pre- andpost-trigger time intervals for all trigger events in the input signalare processed for display. The hold-off algorithm uses a designatedinterval equal to the sum of specified pre- and post-trigger delays todetect trigger events that occur during the designated interval togenerate a flag signal. In response to the flag signal a next displayframe is drawn based upon either the last trigger event to occur duringthe designated interval (flag SET) or the next trigger event that occursafter the designated interval (flag/SET).

1. A method of capturing for display all data associated with alltrigger events within an input signal, comprising the steps of:detecting the trigger events within the input signal to generate atrigger signal; drawing a first display frame for the input signal basedupon a first trigger event within the trigger signal; and drawing asecond display frame for the input signal based upon a subsequenttrigger event within the trigger signal after the first trigger event,the second display frame having minimal overlap with the first displayframe when the subsequent trigger event occurs within a designatedinterval after the first trigger event and the data associated with thetrigger events within the first display frame are captured for display.2. The method as recited in claim 1 wherein the designated intervalcomprises a sum of a desired pre- and post-trigger time intervals, thedesignated interval starting when the first trigger event occurs.
 3. Anapparatus for capturing for display all data associated with all triggerevents within an input signal comprising: means for detecting thetrigger events within the input signal to generate a trigger signal;means for drawing a first display frame for the input signal based upona first trigger event within the trigger signal; and means for drawing asecond display frame for the input signal based upon a subsequenttrigger event within the trigger signal, the second display frame havingminimal overlap with the first display frame when the subsequent triggerevent occurs within a designated interval after the first trigger eventand the data associated with the trigger events within the first frameare captured for display.
 4. The apparatus as recited in claim 3 whereinthe designated interval comprises a sum of a desired pre- andpost-trigger time intervals, the designated interval starting when thefirst trigger event occurs.
 5. A display apparatus, comprising: atrigger detection circuit having as an input an input signal and havingas an output a trigger signal that includes all trigger events withinthe input signal; a first drawing engine having as an input a delayedversion of the input signal and having as an output a first displayframe based upon a first trigger event within the trigger signal; and asecond drawing engine having as an input the delayed version of theinput signal and having as an output a second display frame based upon asubsequent trigger event within the trigger signal, the second displayframe having minimal overlap with the first display frame when thesubsequent trigger event occurs within a designated interval after thefirst trigger event and the data associated with the trigger eventswithin the first display frame are captured for display.
 6. The displayapparatus as recited in claim 5 wherein the designated intervalcomprises a sum of a desired pre- and post-trigger time intervals, thedesignated interval starting when the first trigger event occurs.
 7. Thedisplay apparatus as recited in claim 5 further comprising: an intervaltimer coupled to receive the trigger signal, the interval timer startingwith receipt of the first trigger event and stopping when the designatedinterval times out; a detector having as inputs the trigger signal and astop output from the interval timer, and having a flag output, the flagoutput being set when the subsequent trigger event occurs within thedesignated interval; a trigger store coupled to receive a time stamp foreach trigger event within the trigger signal; and a controller having asinputs the trigger signal, the time stamp from the trigger store for thesubsequent trigger, the flag output from the detector, and the stopoutput from the interval timer, the controller starting the firstdrawing engine upon receipt of the first trigger event and starting thesecond drawing engine upon receipt of the subsequent trigger event sothat, when the flag output is set, the second display frame minmallyoverlaps the first display frame and the data from the delayed inputsignal associated with the trigger events within the first display frameare captured for display.